It has been proposed heretofore to control the heat balance in shaft-type metallurgical furnaces to which a blast gas is supplied, e.g. via tuyeres, especially iron-producing blast furnaces, by adding a coolant to the blast gas.
In general, the heat balance must be controlled in the so-called tuyere arch or notch of the furnace, i.e. the zone of the hearth at which the blast gas (e.g. preheated air) enters the latter to generate the elevated temperatures required for reaction with the coke, the reduction of the iron and the smelting thereof, the iron melt and slag collecting in the bosh below the tuyere arch. Above the tuyere arch the blast furnace charge is generally stacked.
To keep fuel consumption as low as possible and maintain a satisfactory thermal efficiency, it is generally desirable to maintain the temperature of the blast gas immediately upstream from the tuyere arch as high as possible and advantageously above 1100.degree. C.
Blast gas temperature above this value, however, frequently interface with the metallurgical process by generating excessively high flame temperatures, leading to difficulties in controlling the process.
To avoid this problem it has been suggested to feed a coolant with the blast gas into the oven chamber through the tuyeres in controlled quantities designed to maintain the heat balance and to compensate for excessively high blast gas temperatures. Typical coolants are steam, heavy oil, ground coal and natural gas.
Steam is a particularly easy to handle coolant for this purpose but has the disadvantage that it constitutes an oxidizing agent which increases the coke consumption which the heating of the blast gas in the first place attempts to avoid. In other words, steam reduces the effect of heating of the blast gas upon reducing fuel consumption.
Heavy oil and natural gas are relatively expensive commodities for use as coolants and thus their use in blast furnace applications should be avoided especially in this period of depletion of fluid fossil fuel reserves.
While the use of ground coal is less apt to create problems with respect to this latter point, the milling or grinding of the coal is expensive and involves significant wear of the grinders so that maintenance and capital costs are high as well. Furthermore, the abrasion of the walls of the ducts through which the coal is fed to the tuyeres is significant and endangers the useful life of this duct work as well.
Experience has shown that most coals also have a high ash content and a high sulfur content which can detrimentally affect the quality of the pig iron or cast iron produced in the furnace. Furthermore, the use of coal as a coolant has been found to create problems with respect to the chemical properties of the slag and makes it more difficult to control the degree of basicity of the latter within the desired limits.
Thus the art has long sought a solution to the aforementioned problems, i.e. a method whereby the heat balance at the tuyere arch or within the blast furnace or other shaft-type furnace can be controlled without altering physical or chemical properties of the contents of the furnace, at low cost and without high capital expenditure.